A large market exists for disk drives for mass-market computing devices such as desktop computers and laptop computers, as well as small form factor (SFF) disk drives for use in mobile computing devices (e.g. personal digital assistants (PDAs), cell-phones, digital cameras, etc.). To be competitive, a disk drive should be relatively inexpensive and provide substantial capacity, rapid access to data, and reliable performance.
Disk drives typically employ a moveable head actuator to frequently access large amounts of data stored on a disk. One example of a disk drive is a hard disk drive. A conventional hard disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a moveable head for reading and writing data. The HSA forms part of a servo control system that positions the moveable head over a particular track on the disk to read or write information from and to that track, respectively.
Typically, a conventional hard disk drive includes one or more disks in which each disk has a plurality of concentric tracks. Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information may be provided on each disk to determine the position of the head.
Each of the heads typically includes a read/write transducer formed on the trailing surface of a slider. When the disk media is rotated, a thin film of air forms between the disk and an air bearing surface (ABS) of the slider. During operation of the disk drive, the head is said to “fly” over the surface of the disk, with the ABS being disposed just above the disk's surface. The thin film of air formed between the ABS and the disk surface is known as the air bearing. The very small separation distance between the transducer of the flying head and the surface of the disk is referred to as the “fly height”. When the flying head is suspended above the disk in this manner, it is moved by the servo control system over a desired concentric track of the disk to access data stored on that track.
The fly height of the head is a factor affecting the density of magnetic data that can be stored on the disk. In recent years, the magnetic recording industry has strived to increase data storage density by employing various techniques aimed at decreasing the average fly height of the head over the rotating disk. Dynamic fly height (DFH) heads are utilized to fly at increasingly smaller fly heights to increase data storage capacity.
Typically, to control the fly height of a DFH head relative to a disk, power is applied in the form of current to a heater element of the DFH head which causes the DFH head to move closer to the disk. In this way, the DFH head is able to fly at a predetermined distance from the disk in order to read and write magnetic patterns to the disk. As storage capacity has increased, DFH heads are required to fly closer to disks and to maintain smaller more precise distances from the disks.
In order to characterize a DFH head to determine an optimal fly height, testing is performed to characterize the fly height of the DFH head across an applied power range. These characterization methods typically rely on spacing models that are utilized to estimate Head Media Separation (HMS). Typically, a Wallace spacing model is used. The Wallace spacing model, or Wallace spacing loss equation, expresses a relationship between the read-back voltage from the head and head/disk spacing. In particular, the Wallace spacing loss equation describes the amplitude of the read-back signal to the spacing of the head above the recording medium (HMS), which may be modeled with the following exponential:HMS=e−kd where HMS is the fly height of the head above the disk; k is the spatial wavelength between two magnetic data transitions and d is the distance of the head above the magnetic layer in the media. Such a modeling may be used with Longitudinal Magnetic Recording (LMR) channels and also may be used with Perpendicular Magnetic Recording (PMR) channels, provided the frequency range is suitably limited. The HMS quantity, in this context, is not to be thought of as an absolute predictor of the distance of the heads above the magnetic layer of the media (in contrast to the surface thereof), but rather as an indication of change in the fly height of the heads above the magnetic layer of the media. Such an indication of the change in the fly height of heads over the magnetic layer in the spinning media may be determined by determining the HMS for two different wavelengths, which may be derived from two different frequencies of signals written on the disk.
As is known, frequency domain processing of Analog to Digital (ADC) samples is typically carried out to perform narrow band measurements (e.g. burst amplitude detection, HMS, etc.). When used to determine HMS, the frequency domain processing has been carried out on data read from either on dedicated tracks in the data area on which the two signals of different frequencies have been written or on dedicated bursts of differing frequencies in the servo area. Both of these approaches have disadvantages: dedicated data tracks require increased format complexity and are susceptible to thermal decay issues, while dedicated bursts in the servo area reduce the amount of data that can be stored on the disk. Moreover, as the read-back signal from the heads is acquired while servo processing or attempting to determine HMS, it is amplified by a Variable Gain Amplifier (VGA) whose gain may be adjusted at every sample during preamble acquisition, which is not optimal when attempting to determine HMS from multiple samples.
What are needed, therefore, are methods for determining HMS and disk drives configured to determine HMS that do not suffer from the above-described disadvantages.